A mirror is a device that is arranged to reflect light. The term light is understood to include both visible electromagnetic radiation, and other wavelengths of electromagnetic radiation.
An adjustable mirror is a mirror in which the reflective properties of the mirror can be controllably adjusted. For instance, the shape (or apparent shape) of the reflecting surface may be altered, or the position (or apparent position) of the reflecting surface may be altered.
One type of adjustable mirror is the deformable mirror, in which the reflective surface of the mirror can be controllably deformed so as to provide a desired shape. For instance, the reflective surface of a spherical mirror may be controllably deformed so as to alter the radius of curvature of the mirror. Such deformable mirrors are typically used in reflecting telescopes.
U.S. Pat. No. 5,880,896 describes a deformable mirror for use in an optical disc recording/reproducing apparatus. The mirror is adjusted by controllably deforming a flexible member having a reflective surface, the member being deformed by an electrostatic stress.
Such deformable mirrors are susceptible to wear, as the mirror is continually stressed and de-stressed to obtain the desired shape. Further, deforming the reflective surface in the desired manner is difficult to control, and consequently it is relatively expensive to provide a deformable optical mirror of good optical quality.
DE 19710668 describes an alternative adjustable mirror 10, as illustrated in FIGS. 1A and 1B. The mirror system 10 comprises a membrane 12 filled with a fluid 14. The pressure of the fluid 14 within the membrane 12 is controlled by means of a pump 16. The membrane 12 acts as a variable lens, with the shape (and hence power) of the lens varying in dependence upon the pressure of the fluid 14. FIG. 1A shows the fluid 14 at a low pressure i.e. with the membrane 12 forming a bi-concave lens. FIG. 1b shows the fluid 14 at a higher pressure, with the membrane 12 forming a bi-convex lens.
Attached to the central portion 18 of one surface of the membrane 12 is a rigid convex reflective surface 20. The mirror 10 formed by the membrane 12 and the reflecting surface 20 is adjusted by varying the pressure of the fluid 14. Consequently, the power of the lens formed by the membrane 12 is altered, and hence the apparent curvature of the mirror (i.e. the total power of the mirror which is the result of the lens shape created by the membrane 12 and the curved reflective surface 20) is adjusted. The disadvantage of such a system is that, due to the movement of the membrane surface and the reflective surface it is difficult to maintain good optical properties and it is susceptible to vibrations. Further, it is susceptible to mechanical wear.
It is an aim of embodiments of the present invention to provide an adjustable mirror that addresses one or more problems of the prior art, whether referred to herein or otherwise. It is also an aim of the present invention to provide optical devices incorporating such improved adjustable mirrors, and methods of manufacturing such improved adjustable mirrors and such optical devices.
It is an aim of particular embodiments of the present invention to provide an adjustable mirror in which the optical path is relatively unsusceptible to mechanical wear and tear during operation.